REVIEW QUESTIONS

1a. What are the wavelengths of solar radiation that are harmful to human systems?

1b. What types of damage can this radiation generate in humans?

1c. How does ozone protect us from damaging radiation?

 

2a. How is ozone produced? Is the production slow or fast? Explain.

2b. Where in the atmosphere does ozone naturally form and why?

2c. Is ozone a stable molecule? Explain.

2d. Is there a pattern(s) to the distribution of ozone in the atmosphere? What processes control the pattern(s)?

3a. What natural processes or substances can destroy ozone? How do they do this? How about manmade substances?

3b. Explain why CFCs have proven to be so useful to our society.

3c. CFCs are very stable and extremely long lived in the troposphere but are eventually destroyed in the stratosphere. Explain how CFCs are eventually destroyed. Why does it take such a long time?

3d. Why is the destruction of CFCs in the upper stratosphere a concern for stratospheric ozone?

 

4a. What is the Antarctic ozone hole?

4b. Is the ozone hole located over Antarctica year round? Explain.

4c. Is there an Arctic ozone hole?

4d. Why don't we see ozone holes over North America or the tropics?

5a. What steps have been taken to decrease the amounts of CFCs introduced into the atmosphere?

5b. If we stopped using CFCs today would the effect on stratospheric ozone vanish immediately? Explain.

5c. How long before stratospheric CFC levels begin to decrease? What will happen eventually to Antarctic and Arctic ozone levels?

ANSWERS

1a. Solar radiation with wavelengths shorter than those in the visible region (below 400 nm) are harmful to human systems.

1b. Specifically, ultraviolet radiation can cause sunburn and promote DNA damage that can lead to dangerous forms of skin cancer (basal, squamous, and melanoma).

1c. Ozone protects us from damaging radiation by absorbing harmful ultraviolet radiation (UV photons). Ultraviolet radiation is effectively "screened out" in the stratosphere and almost none reaches the surface.

2a. Ozone is produced when molecular oxygen (O2) is broken apart into oxygen atoms (O) by intense ultraviolet radiation (wavelength less than 240 nm) high in the atmosphere. These oxygen atoms then react with other O2 molecules to form ozone (O3) molecules, usually in the presence of a third body molecule (M), which carries away the extra energy. This process can be written as

O2 + hc/ --> O + O ( <240 nm)

where hc/ represents the ultraviolet ray or photon. The rate at which ozone is formed is slow, since there isn't a lot of solar energy at wavelengths less than 240 nm.

2b. About 90% of the ozone in our atmosphere is contained in the stratosphere, the region from10 to 50km above Earth's surface. At these altitudes there is enough of the necessary oxygen molecules and the necessary ultraviolet radiation to form ozone.

2c. Ozone is a highly reactive molecule and is rapidly converted to oxygen when it encounters a UV photon. In this respect it is more unstable than oxygen.

2d. There are patterns of distribution in ozone based on season and latitude, as well as altitude. These patterns are controlled by the angle (and hence intensity) of solar radiation and by transport processes that redistribute ozone between the tropics and poles.

3a. The absorption of a UV photon by ozone in the stratosphere leads to the natural destruction of ozone. The reaction is given by

O3 + hc/ ( <400 nm) --> O2 + O

3b. CFCs have made safe (nontoxic, inflammable) refrigerants and spray can propellants.

3c. CFCs can only be destroyed by extremely energetic ultraviolet radiation, such as is found above most of the ozone layer. CFCs are broken down by UV photolysis, but the process takes a long time since it takes decades to cycle all of the air in the troposphere through the upper stratosphere.

3d. The photolysis of CFCs generates a highly reactive chlorine atom that can attack ozone or form compounds that destroy ozone.

4a. The Antarctic ozone hole is a region of very large ozone losses over Antarctica centered on the South Pole that occur in the southern hemisphere spring.

4b. The hole is a southern hemisphere spring phenomenon appearing in September and breaking up at the end of October.

4c. While there is not an symmetrical Arctic "hole" as seen in the Antarctic, there have been severe spring ozone losses recorded in the last several years for the north polar region.

4d. The CFCs that destroy ozone are distributed throughout the atmosphere. However, the holes form as a result of exceptionally cold temperatures which occur during the winter season over the polar regions. These cold temperatures isolate air in the polar regions from ozone rich air at lower latitudes and allow formation of polar stratospheric clouds that enhance ozone destructive processes.

5a. Concern for the health of the stratospheric ozone layer led to an international agreement in 1987, the landmark Montreal Protocol, that restricted CFC production. This international agreement and its amendments have led to a curtailment of CFC production around the world.

5b. After entering the lower stratosphere, the CFCs can either be mixed to higher latitudes, or slowly carried into the upper stratosphere (shown by the blue arrows) where they can be broken down by the Sun's UV radiation. Because CFCs are broken down by UV radiation at higher altitudes, CFC concentrations decrease with altitude. It takes about a full year for the average CFC molecule to get to the upper stratosphere from the tropical upper troposphere. However, most of this air entering the stratosphere gets recycled back into the troposphere before reaching the upper stratosphere. It takes a few decades or more to cycle all of the air in the troposphere through the upper stratosphere. This slow circulation of CFCs through the upper stratosphere means that it will take decades to cycle all of the CFCs through the upper stratosphere. Even if we banned all CFCs today it will take time to destroy all CFCs put into the atmosphere previously.

5c. Stratospheric CFC levels should begin to decrease within the next few years. As these chlorine and bromine levels begin to decrease, Antarctic and Arctic ozone levels should begin to recover.